1. Field of the Invention
The invention relates to a device as well as a method for blocking a stack of stacked objects, for example stacked newspapers, magazines, or the like.
2. Description of the Prior Art
A device of this kind is known (WO-96/33118) and has a lifting table by which a stack of stacked objects lying thereon can be pressed against two flaps serving as counterpressure plates. During the compression of the stack, the stack is blocked by charging electrodes located laterally with respect to the stack. Since the paper stack has a higher dielectric constant than air, the electrical field concentrates in the X stack. In addition to this concentration, the field is concentrated in the superfluous air inclusions in the stack, so that the field force effect in the stack and in the charge accumulated at the surface compresses the air in the stack. As a result, the adhesion of the objects is decreased, so that the stack is mechanically and firmly blocked and consequently is secured. During the compression of the stack, charging electrodes located laterally next to the stack, depending on the height of the stack, can be switched on.
Similar devices are known for blocking stacks (DE-44 34 946 A1 and DE-44 41 431 A1) that have a transport table, with lateral charging electrodes located on the transport table, said electrodes extending perpendicularly to the transport direction of the stack and extending perpendicularly to the transport table for the entire height of the stack. Opposite the transport table is another grounded charging electrode that contacts the stack from above. This charging electrode can be provided with one or more rollers that are pressed from above against the stack.
Finally, a device is known for aligning, pressing, and blocking packets of loosely stacked printed products (G 295 06 231 U1). This device consists of a packet conveyor with a front packet stop that can be raised and lowered, two packet aligning plates located laterally with respect to the packet conveyor and capable of being brought together or moved apart, and a pressing device above that can be raised and lowered and can be placed between the packet alignment plates on top of the packet to be pressed. It has a charging assembly for electrostatic blocking of the printed products, whose charging electrodes are connected to the packet alignment plates located laterally with respect to the packets and to the pressing device, so that the packets are charged with a voltage on their lateral wall areas and their tops.
The object of the invention is a device for blocking a stack of stacked objects, stacked newspapers, magazines, or the like for example, by which the stack to be blocked can be charged very effectively.
This object is achieved in a device and a method.
The device according to the invention has a pressing device with two pressing elements opposite one another, between which a stack can be pressed. The charging electrodes for charging this stack are integrated into the pressing elements opposite one another, so that the latter are pressed against the stack when compressing the stack. In addition, charging electrodes can also be provided in the side walls to which a high positive or negative voltage is applied, in any case with a different polarity than on the upper charging electrodes.
The device according to the invention is very simple in design, because the charging electrodes, which are known from the prior art and as a rule are located laterally next to the stack to be blocked, can be completely eliminated, since they are integrated into the pressing elements.
The pressing elements opposite one another thus form, in the manner of a plate capacitor, an electrical field that runs essentially parallel to the pressing direction and passes perpendicularly through the objects to be stacked, for example newspapers, magazines, or the like, so that the desired displacement charges accumulate on their surfaces and build up a capacitive displacement field that opposes the electrical field applied externally. Since the directions of the electrical field applied externally and of the displacement field induced in the stacked objects run parallel to one another, during pressing a capacitive displacement current flows so that the charging of the stack is extremely effective and by which high adhesion forces are produced. The displacement current is produced by the air being forced out of the stack.
Although it is known (WO 96/33118) to ground the supporting table that serves as a pressing element and the flaps located opposite the supporting table or to apply the same potential, so that here as well, charging electrodes are provided above and below the stack for blocking the latter. Since the supporting table and flaps are at the same potential however, and additional charging electrodes are located laterally, a field pattern is produced that passes diagonally through the stacked objects.
Preferably, the stack is charged by the device according to the invention during pressing at the same time, since as a result of the compression of the stack, its dielectric constant is increased during pressing, so that a correspondingly high electrical field forms between the charging electrodes.